fix README for python3 (#7947)

lol

Author: alex

README.rst

@@ -30,9 +30,9 @@ key derivation functions. For example, to encrypt something with
     >>> f = Fernet(key)
     >>> token = f.encrypt(b"A really secret message. Not for prying eyes.")
     >>> token
-    '...'
+    b'...'
     >>> f.decrypt(token)
-    'A really secret message. Not for prying eyes.'
+    b'A really secret message. Not for prying eyes.'
 
 You can find more information in the `documentation`_.